Field of the Invention
The present invention relates to a hollow fiber membrane module suitable as a final filter that removes particulates in water to be treated in a ultrapure water manufacturing process.
Related Background Art
In a line where ultrapure water used to manufacture electronic and electric components such as semiconductors and display elements, a membrane filter module is used as a final filter that removes particulates from ultrapure water manufactured using microfiltration membranes and an ion exchange resin or reverse osmosis membranes immediately before the ultrapure water is supplied to a use point. As a membrane filter module for this application, an external-pressure hollow fiber membrane module that supplies raw water to the outside of a hollow fiber membrane to filter the water is used because the hollow fiber membrane module has the advantage of enabling an increase in filtration flow rate per module.
With a recent increase in the amount of production of electronic and electric products, the amount of ultrapure water used has been increasing. On the other hand, miniaturization of semiconductor elements has been progressing. In such circumstances, there has been a demand for a hollow fiber membrane module having a high filtration capability, that is, a hollow fiber membrane module which has a high filtration flow rate per module and which enables particulates to be reliably removed.
However, in the conventional external-pressure hollow fiber membrane module, when the amount of water supply is increased to increase throughput per unit time, water flows through the hollow fiber membrane module at a high speed. Thus, a phenomenon in which the hollow fiber membrane oscillates violently may occur and last to fatigue and break the hollow fiber membrane. As a result, particulates flow into filtrate to degrade water quality. As a method for preventing damage done to the hollow fiber membranes by a water flow, a method has been proposed in which a protective straightening cylinder is provided around a bundle of hollow fiber membranes (see Japanese Examined Patent Publication No. H07-102307 and Japanese Patent Application Laid-Open No. 2000-37616).
Furthermore, when a bundle of hollow fiber membranes is housed in a housing and fixed at opposite ends of the bundle using adhesive fixation layers, the density distribution of the hollow fiber membranes is likely to be biased. When the hollow fiber membranes are formed with a large bias retained, a bias of a water flow occurs in the module. Thus, the hollow fiber membranes oscillate and are consequently broken. To prevent such a bias of the density distribution of the hollow fiber membranes, the housing may be filled with the bundle of hollow fiber membranes as densely as possible to eliminate gaps that may lead to a bias. However, such a method poses the following problem. Even channels of water flowing among the hollow fiber membranes and through the housing are narrowed to increase resistance to the water flow. Thus, operation power is increased, and moreover, when the external-pressure configuration is used for filtration, the hollow fiber membranes come into tight contact with one another to reduce an effective membrane surface for filtration. Accordingly, a rise in transmembrane pressure difference is accelerated.
To solve this problem, for example, members serving as spacers (partitioning plates) may be inserted into the bundle of hollow fiber membranes (see Japanese Patent Application Laid-Open No. 2011-25156). However, in such a method, the partitioning plates serving as the spacers and the hollow fiber membranes come into contact one another, and the hollow fiber membranes may be damaged and fail to provide an original separation capability or may be broken.
On the other hand, a hollow fiber membrane module is known in which an insertion object that prevents a bias of the density distribution of the hollow fiber membranes is fixedly inserted into an adhesive fixation layer (see WO97/10893 and Japanese Patent Application Laid-Open No. 2012-45453). In such a hollow fiber membrane module, since the insertion object is present in the adhesive fixation layer, the contact between the spacers and the hollow fiber membranes as described above is avoided, while the hollow fiber membranes pushed away by the insertion of the insertion object have smaller spaces between the membranes. As a result, the hollow fiber membranes present in the areas other than the area into which the insertion object has been inserted can be made uniform in density distribution.